1. Field of the Invention
The present invention relates to a method and a device for controlling the start time of a vehicle heat engine.
The invention also relates to a starter-alternator which can carry out the method and comprises said device.
2. Description of Related Art
Energy saving and pollution reduction considerations, particularly in urban surroundings, have prompted automotive vehicle manufacturers to equip their models with an automatic heat engine stop/restart system such as the system known as the “Stop and Go” system.
As discussed in VALEO EQUIPEMENTS ELECTRIQUES MOTEUR's French patent document FR 2 875 549, vehicles are rendered capable of functioning in “Stop and Go” mode by dint of a reversible electrical machine, or starter-alternator, coupled to a heat engine supplied by an AC/DC converter in “starter” mode.
Under certain conditions, using a starter-alternator in the “Stop and Go” functional mode consists of causing the heat engine to stop completely when the vehicle itself is stopped, then restarting the heat engine subsequent, for example, to an action by the driver which is interpreted as a restart command.
A typical “Stop and Go” situation is that of stopping at a red light. When the vehicle stops at the light, the heat engine is automatically stopped, then when the light turns green, the engine is restarted using the starter-alternator following detection by the system of the clutch pedal being depressed by the driver, or of any other action which is interpreted as meaning that the driver intends to restart the vehicle.
Clearly, the automatic restart function carried out by a starter-alternator system is a function which must be as transparent as possible for the driver of the vehicle.
In starter-alternators constituted by a polyphase rotary electrical machine, the phase currents and the excitation current are generally supplied simultaneously by the power circuits at the instant of restarting.
US. Pat. No. 6,335,609 states that under such circumstances, the engine torque can only be produced with a perceptible delay.
That delay is due to a magnetic flux being established in the rotor, and it proposes pre-fluxing the inductor before the phase currents are established, in order to reduce the time necessary for the heat engine to attain a predetermined rotation speed.
However, the method is carried out by controlling the excitation current for a fixed period, and it does not appear to be adapted to starter-alternators which are supplied by an on-board variable voltage electrical network of the “14+X” type.
As a result, there is a need for a method and a device which can keep the start time within limits which are acceptable to the driver in the case of an automatic stop/restart system architecture of the micro-hybrid type, where the voltage of the on-board electrical is network depends on the charge state of the ultracapacitor.